The present invention relates to a storage system.
Generally, storage systems comprise a cache memory. Cache memory is used to temporarily memorize data to be written to at least one of a plurality of storage devices in a storage system in accordance with a write command received from the upper level device, or data read from one of a plurality of storage devices in accordance with a read command.
Power supplied to the cache memory may be lost if the power source of the storage system fails. In this case, if the cache memory is volatile memory, the data memorized in the cache memory will be lost.
For example, disclosed in Literature 1 (U.S. Pat. No. 5,586,291) and Literature 2 (Japanese Patent Application Laid-open number H6-222988) respectively is the provision of non-volatile memory in addition to volatile cache memory, and the storage of data in non-volatile memory. Moreover, disclosed in Literature 3 (U.S. Pat. No. 6,539,456) and Literature 4 (US Patent Publication number 2004/64647) respectively is the utilization of non-volatile memory as cache memory.
Even though called non-volatile, the non-volatile memory disclosed in Literature 1 and Literature 2 is memory that can hold data by receiving power supplied from a battery. For that reason, the technology disclosed in this literature requires a battery in order to continue to hold data in non-volatile memory when the power source to the storage system fails.
Moreover, in literature 2 a redundant configuration is disclosed in which, when one system fails, the other system copies data memorized in non-volatile memory to a direct access storage device such as a tape or disk drive device. In addition, there is the risk that data may be lost if the other power source fails during that data copy. For this reason, power supplied from a battery is used to copy data from non-volatile memory to a direct access storage device. However, a large capacity battery that can continue to supply power for a long time is necessary because the access speed of direct access storage devices is slow.
Thus, the technology disclosed in literature 3 and literature 4 is applied to the storage system. Specifically, rather than volatile memory, this method makes the cache memory of the storage system into a type of non-volatile memory that can continue to memorize data even without power supplied from a battery.
However, this type of non-volatile memory generally tends to degrade with the number of update times (number of erase times). Even when non-volatile memory is used as cache memory in a small-scale device such as personal computers, updating may not cause much of a problem with shortening the lifespan of the non-volatile memory. However, in large-scale devices such as storage systems, updating cache memory occurs more frequently than in the small-scale devices described above, and therefore, the shortened lifespan corresponding to the number of update times is a problem.